Such a method and system are known from US patent application US 2007/0133354, International patent applications WO2007068891 and WO2007068891 and from the paper “Pushing the Vibrator Envelope—How Low Can We Go” presented by Z. Wei et al at the EAGE 69th Conference & Exhibition held in London, UK from 14 to 17 Jun. 2007.
In this paper the following conclusions are drawn.
The reaction mass peak-to-peak stroke and the peak-decoupling force are two key factors that limit the energy that can be transmitted into the earth by a P-wave vibrator. The reaction mass peak-to-peak stroke is more dominant at very low frequencies. As frequency is increased the peak-decoupling force limits the vibrator output. The boundary frequency between these two factors falls between 4 and 5 Hz depending on the weight of the reaction mass. The heavier the reaction mass, the lower the boundary energy becomes. A heavier mass helps the vibrator produce more fundamental force energy and reach the target force quickly.
The paper “Low Frequency vibroseis data with maximum displacement sweep” published by C. Bagain in the Leading Edge, Society of exploration geophysics, Tulsa, Okla., US, Vol 27, No. 5, 1 May 2008, pages 582-584, 586, 5, XP001512169, ISSN:1070-458X describes a maximum displacement (MD) sweep, which uses measurements to specify the seismic vibrator behaviour at low frequencies (typically 1-6 Hz). Subsequently, in the known method a sweep function is developed that is required to generate a flat output spectrum; typically, this sweep function is the inverse of the measured response.
Other Maximum Displacement (MD) sweep methods which use measurements to specify the seismic vibrator behaviour at low frequencies are known from US patent applications US2006/018192 and 2007/133354 and UK patent 2387226.
It is an objective of the present invention to provide a method and system, which generate an enhanced low frequency content in a seismic signal in a more efficient and accurate manner than the prior art methods and systems, and which obviate the time consuming and expensive requirement to use measurements to specify the seismic vibrator behavior at low frequencies associated with the known Maximum Displacement (MD) sweep methods.